Urea kinetic modeling has become the standard by which dialysis dose is measured. Accurate calculation requires measuring residual renal function (24-48 hour urine), which is difficult in small children and clearance by the dialysis procedure. Dialysis clearance is dependent on determining the amount of urea removed. The hypothesis is that a predictive mathematical model can be developed that would obviate the need for collection of urine to determine residual function and would take into account urea rebound. Such a model would result in a more accurate measure of dialysis "dose" in children.